It is known in the art to provide automatically or semi-automatically operated means for advancing and retracting an apparatus element.
A ready example is the fluid-powered actuators provided for opening and closing gate valves from remote sites.
One example of a prior art fluid-pressure actuated gate valve operator is disclosed in working association with a gate valve in the prior U.S. Patent of McGee, No. 3,789,875, issued Feb. 5, 1974.
It is not uncommon in such valve actuators, where a reciprocating stem passes out of the bonnet of the valve housing through a packing annulus, to provide a metal-to-metal seal between the stem and the bonnet, to permit the valve packing to be changed while the valve remains installed and in service.
Such a metal-to-metal seal is shown, though unnumbered, in FIG. 3 of the aforesaid McGee patent, as mating frusto-conical surfaces on the stem 254 and the housing bonnet 258, near the lower end of the stem 254.
Another example is shown in the prior U.S. Patent of Dyson, No. 3,219,056, and the prior U.S. Patents of Donneley, No. 551,370, Scott, No. 667,797 and Schmitt, No. 738,362 show others.
It is also known to provide a dashpot or shock absorber for the stem of a fluid-powered actuator to guard against undue loading.
The aforesaid Dyson patent shows one such device, and the prior U.S. Patents to Collin et al., No. 534,360, Parsons et al., No. 846,692 and Holmes, No. 1,778,191 show others.